A metal wiring is used in semiconductor devices, for example, to obtain electrical connection between elements such as transistors and diodes. In addition, to achieve multifunctionality and high integration of semiconductor devices, substrate bonding may be used. In substrate bonding, semiconductor devices formed on different substrates are stacked and bonded together to be integrated. For example, multifunctionalization of semiconductor devices can be realized by bonding semiconductor devices having different functions. Further, for example, by bonding semiconductor devices having the same type of functions, the semiconductor devices can be highly integrated. In substrate bonding, for example, the metal wirings formed on a surface of each substrate are directly bonded to each other to obtain electrical connection between the stacked semiconductor devices.
Forming a metal wiring at low cost is desired to reduce the cost of a semiconductor device. Further, a method for forming a metal wiring which can realize substrate bonding at low cost is desired.